Polymerization of polyethylene having high molecular weight

ABSTRACT

The present invention includes a bimodal polyethylene polymerization process wherein metallocene catalyst to is used to adjust the hydrogen response of a Ziegler-Natta catalyst. The polymerization may be carried out in a single reactor or in two or more reactors in series, preferably two or more continuously stirred tank reactors in series. In an embodiment having two or more reactors, the Zeigler-Natta catalyst is added to a first reactor and the metallocene catalyst is added to a downstream reactor. In another embodiment having two or more reactors, the Zeigler-Natta catalyst and metallocene catalyst are added to the same reactor, preferably an upstream reactor. A preferred Zeigler-Natta catalyst comprises TiCl 4 , and a preferred metallocene catalyst comprises bis(cyclopentadienyl) titanium dichloride.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Not applicable.

REFERENCE TO A MICROFICHE APPENDIX

[0003] Not applicable.

FIELD OF THE INVENTION

[0004] The present invention relates to the use of Zeigler-Natta andmetallocene catalysts in a polymerization process to produce highdensity polyethylene (HDPE), and more particularly to the use of ametallocene catalyst to adjust the hydrogen response of a Zeigler-Nattacatalyst in the production of HDPE preferably having bimodal physicalproperties.

BACKGROUND OF THE INVENTION

[0005] Bimodal HDPE comprises a low molecular weight (LMW) fraction anda high molecular weight fraction (HMW), typically referred to as abimodal molecular weight distribution (MWD). Bimodal HDPE combines theadvantages of low molecular weight polyethylene such as ease ofprocessing and high melt flow index with the physical propertyadvantages of high molecular weight polyethylene such as good impactresistance and good slow crack growth resistance. In making bimodalHDPE, the relative proportion of the low and high molecular weightfractions may be adjusted (as measured by the MWD) to provide HDPEhaving desired physical properties. For example, broadening the MWD ofan HDPE polymer typically tends to improve the shear response of thepolymer, thereby improving processing behavior in extrusion processes(such as in blown film, sheet, pipe and blow molding equipment).

[0006] The MWD may be determined by means of a curve obtained by gelpermeation chromatography (GPC). For a polymer having a bimodal MWD, theGPC curve may resemble a peaked bell curve having a “shoulder” on thehigh molecular weight side of the peak or by two distinct peaks.Generally, the MWD is defined by a parameter known as the polydispersityindex (D), which is the ratio between the average molecular weight byweight (Mw) and the average molecular weight by number (Mn), i.e.,D=Mw/Mn. The polydispersity index (D) provides a measure of the width ofthe molecular weight distribution for a polymer composition.

[0007] Bimodal HDPE is typically produced in a multi-stagepolymerization process, for example polymerization of the low molecularweight fraction in a first stage and polymerization of the highmolecular weight fraction in a second stage (or it can be the reverse).The multi-stage polymerization may be carried out in a single reactor orin two or more reactors in series, and suitable reactor types includestirred tanks, loop reactors, gas phase reactors, tubular reactors,autoclaves, and combinations thereof. Differing polymerizationconditions may be achieved in the stages by varying parameters such asthe reaction conditions (e.g., time, temperature, pressure, etc.) andthe type and amount of reactants (e.g., monomer, co-monomers),catalysts, cocatalysts, chain transfer/termination agents (e.g.,hydrogen), and the like.

[0008] Conventional, bimodal HDPE polymerization processes typically useone or more Zeigler-Natta catalysts wherein the production of lowmolecular weight and high molecular weight fractions is achieved byadjusting the hydrogen response of the Zeigler-Natta catalyst—anincrease in hydrogen response producing a lower molecular weight polymerand conversely a decrease in hydrogen response producing a highermolecular weight polymer. More specifically, hydrogen serves as a chaintermination agent for the Zeigler-Natta catalysts. Increasing theconcentration of hydrogen in the polymerization reaction leads toincreased termination of the polymer chains (i.e., shorter chainlengths), which produces a lower molecular weight polymer. Conversely,decreasing the concentration of hydrogen in a polymerization reactionleads to decreased termination of the polymer chains (i.e., longer chainlengths), which produces a higher molecular weight polymer. A common wayof adjusting the hydrogen concentration (and thus the hydrogen response)in a conventional bimodal polymerization process is to vent hydrogenfrom a reactor, which wastes valuable reactants such as monomers andresults in increased operating costs. The present invention provides animproved process for production of bimodal HDPE wherein the wastefulventing of hydrogen is substantially reduced or eliminated.

SUMMARY OF THE INVENTION

[0009] The present invention includes a bimodal polyethylenepolymerization process wherein metallocene catalyst to is used to adjustthe hydrogen response of a Ziegler-Natta catalyst. The polymerizationmay be carried out in a single reactor or in two or more reactors inseries, preferably two or more continuously stirred tank reactors inseries. In an embodiment having two or more reactors, the Zeigler-Nattacatalyst is added to a first reactor (in this case the LMW reactor) andthe metallocene catalyst is added to a downstream reactor. In anotherembodiment having two or more reactors, the Zeigler-Natta catalyst andmetallocene catalyst are added to the same reactor, preferably anupstream reactor. In an embodiment, the Zeigler-Natta catalyst comprisesTiCl₄ and the metallocene catalyst comprises bis(cyclopentadienyl)titanium dichloride.

[0010] According to another embodiment of the invention, a polyethylenepolymerization process comprises (a) polymerizing polyethylene in thepresence a Zeigler-Natta catalyst for a period of time, and (b) adding ametallocene catalyst and continuing the polymerization for an additionalperiod of time. Preferably, the polymerization process producespolyethylene having a bimodal molecular weight distribution.

DESCRIPTION OF THE DRAWINGS

[0011] The invention, together with further advantages thereof, may bestbe understood by reference to the following description taken inconjunction with the accompanying drawing in which:

[0012] FIGS. 1-7 are GPC curves for the polymer produced in Examples1-7, respectively.

[0013]FIG. 8 is a GPC curve for the polymer produced in ComparativeExample 2.

DETAILED DESCRIPTION OF THE INVENTION

[0014] In an embodiment of the present invention, polyethylene having abimodal MWD is polymerized in a two-stage polymerization reaction. Inthe first stage, ethylene monomer is polymerized into polyethylene inthe presence of a Zeigler-Natta catalyst such that the resultingpolyethylene fraction is of relatively low molecular weight incomparison to the polyethylene fraction produced during the secondstage. As discussed previously, hydrogen is used as a chain terminationagent with the Zeigler-Natta catalyst to regulate the extent ofpolymerization occurring during the first stage. Polymerization reactionconditions such as temperature, pressure, and reaction time are selectedand suitable such that the resulting polyethylene fraction displays thedesired relatively low molecular weight.

[0015] Upon achieving a desired fraction of relatively low molecularweight polyethylene, the second stage of the polymerization reaction iscarried out by using a metallocene catalyst to lower the hydrogenresponse of the Zeigler-Natta catalyst, whereby the polymerizationreaction is continued to produce a polyethylene fraction having arelatively high molecular weight in comparison to the polyethylenefraction produced during the first stage. The amount of metallocene usedis such that the mass ratio of metallocene catalyst to Zeigler-Nattacatalyst (i.e., g metallocene/g Zeigler-Natta) is in the range of fromabout 0.1 to about 2.0, preferably from about 0.25 to about 1.5, morepreferably about 0.5-1.0. During the second stage, additional ethylenemonomer and/or a comonomer such as butene may be added to thepolymerization reaction. In a continuous process, typically noadditional hydrogen is added during the second stage, as increasedhydrogen concentration is generally detrimental to producing highermolecular weight polyethylene, as noted above. Polymerization reactionconditions such as temperature and residence time are selected andsuitable such that the resulting polyethylene fraction displays thedesired relatively high molecular weight and the resulting polyethyleneproduct displays the desired bimodal MWD. Upon completion of thepolymerization reaction, the polyethylene product is recovered.

[0016] The resulting polyethylene produced according to the presentinvention is HDPE having a bimodal MWD in the range of from about 10 toabout 35 and more preferably from about 15 to about 30 and melt flowindex (MI5) in the LMW reactor in the range of from about 100 to about3000 dg/min, more preferably from about 250 to about 2000 dg/min, andmost preferably from about 400 to about 1500 dg/min. In the HMW reactorthe melt flow index is in the range of from about 0.1 to about 10dg/min, more preferably from about 0.2 to about 5 dg/min, and mostpreferably from about 0.3 to about 2.0 dg/min. The final density of thebimodal polyethylene is in the range of from about 0.930 to about 0.970g/cc, more preferably in the range of from about 0.940 to about 0.965g/cc, and most preferably from about 0.945 to about 0.960 g/cc. HDPEproduced in accordance with this invention is useful, for example, inextrusion and molding processes to produce a variety of end use productssuch as fibers, webs (both woven and nonwoven), films (both blown andcast), pipe, containers, component parts, and the like.

[0017] The polymerization process of the present invention may beperformed either batch wise or continuously. The polymerization processmay be conducted in a single reactor, as described in the Examplesbelow, or preferably is conducted in two or more reactors connected inseries, more preferably as a continuous process conducted in twocontinuously stirred tank reactors connected in series. In a singlereactor, preferably the Zeigler-Natta catalyst is added to the reactorduring the first stage, and the metallocene catalyst is added to thereactor during the second stage. Alternatively, the Zeigler-Nattacatalyst and metallocene catalyst may be added simultaneously during thefirst stage, with the hydrogen response of the Zeigler-Natta catalystdecreasing as the reaction proceeds to the second stage. With two ormore reactors connected in series, and more specifically with twocontinuously stirred tank reactors connected in series and operatingcontinuously, the Zeigler-Natta catalyst is preferably added to thefirst (i.e., upstream) reactor wherein the first stage polymerization isperformed, and the metallocene catalyst is added to the second (i.e.,downstream) reactor wherein the second stage polymerization isperformed. Optionally, reactants may be recycled from the second reactorto the first, in which case metallocene catalyst will be introduced intothe first reactor regardless of where virgin metallocene is initiallyadded to the reactor system. Alternatively, the Zeigler-Natta catalystand metallocene catalyst can be added simultaneously to the upstreamreactor, with the hydrogen response of the Zeigler-Natta catalystdecreasing as the catalyst travels to the second reactor. Where addedsimultaneously, preferably the Zeigler-Natta and metallocene catalystsare premixed and injected into the reactor together. The catalysts, andpreferably the Zeigler-Natta catalyst, may be prepolymerized to improvethe performance of the catalysts prior to being added to the reactors asdescribed above. Generally, prepolymerization is carried out bycontacting a small amount of monomer with the catalyst after thecatalyst has been activated, for example for example inprepolymerization reactor located upstream of the first reactor.

[0018] Any conventional Zeigler-Natta catalyst and mixtures thereof(hereafter may also be referred to as “Ziegler-Natta catalysts” or“Ziegler-Natta catalysts systems”) suitable for polymerizingpolyethylene homopolymers and/or copolymers may used in performing thepresent invention. Ziegler-Natta catalysts systems may include aconventional Ziegler-Natta catalyst, a support, one or more internaldonors, and one or more external donors. Conventional Ziegler-Nattacatalysts typically comprise a transition metal compound (or mixturesthereof) that may be described by the general formula:

MR⁺ _(x)

[0019] where M is a transition metal, R⁺ is a halogen or ahydrocarboxyl, and x is the valence of the transition metal. Preferably,M is a group IVB metal, more preferably titanium, chromium or vanadium,and most preferably titanium. Preferably, R⁺ is chlorine, bromine, or analkoxy, more preferably chlorine or an ethoxy, and most preferablychlorine. Preferred transition metal compounds are TiCl₄, TiBr₄,Ti(OC₂H₅)₃Cl, Ti(OC₃H₇)₂Cl₂, Ti(OC₆H₁₃)₂Cl₂, Ti(OC₂H₅)₂Br₂,Ti(OC₁₂H₂₅)Cl₃, and combinations thereof, and most preferably titaniumtetrachloride (TiCl₄). No restriction on the number of transition metalcompounds is made so long as at least one transition metal compound ispresent. The transition metal compound is typically supported on aninert solid such as a metal hydride and/or metal alkyl, preferably amagnesium compound such as magnesium halides, dialkoxymagnesiums,alkoxymagnesium halides, magnesium oxyhalides, dialkylmagnesiums,magnesium oxide, magnesium hydroxide, magnesium carboxylates, and morepreferably magnesium dichloride or magnesium dibromide. Typicalmagnesium levels are from about 12% to about 20% by weight of catalyst.Silica may also be used as a support. The supported Zeigler-Nattacatalyst may be employed in conjunction with a co-catalyst, preferablyan organoaluminum compound; more preferably an alkylaluminum compound ofthe formula AlR{circumflex over ( )}3 where R{circumflex over ( )} is analkyl having 1-8 carbon atoms and each R being the same or different,for example, triethylaluminum (TEAl), trimethyl aluminum (TMA) andtriisobutyl aluminum (TiBAL); and most preferably TEAl. Suitableconventional Ziegler-Natta catalysts are disclosed in, for example, U.S.Pat. No. 4,701,432 (in particular, see column 5 line 27 to column 6 line5); 4,987,200 (in particular, see column 27 line 22 to column 28 line17); U.S. Pat. Nos. 3,687,920; 4,086,408; 4,376,191; 5,019,633;4,482,687; 4,101,445; 4,560,671; 4,719,193; 4,755,495; and 5,070,055,each of which is incorporated by reference herein in its entirety.

[0020] Conventional Ziegler-Natta catalysts may be used in conjunctionwith one or more internal electron donors. These internal electrondonors are added during the preparation of the catalysts and may becombined with the support or otherwise complexed with the transitionmetal halide. A suitable Ziegler-Natta catalyst containing adiether-based internal donor compound is that available as Mitsui RK-100and Mitsui RH-220, both manufactured by Mitsui Chemicals, Inc., Japan.The RK-100 catalyst additionally includes an internal phthalate donor.

[0021] Conventional Ziegler-Natta catalysts may also be used inconjunction with one or more external donors. Generally such externaldonors act as stereoselective control agents to control the amount ofatactic or non-stereoregular polymer produced during the reaction, thusreducing the amount of xylene solubles. Examples of external donorsinclude the organosilicon compounds such as cyclohexylmethyldimethoxysilane (CMDS), dicyclopentyl dimethoxysilane (CPDS) anddiisopropyl dimethoxysilane (DIDS). External donors, however, may reducecatalyst activity and may tend to reduce the melt flow of the resultingpolymer.

[0022] Any metallocene catalyst or mixtures thereof (hereafter may alsobe referred to as “metallocenes”, “metallocene compounds”, or“metallocene catalysts systems”) suitable for polymerizing polyethylenehomopolymers and/or copolymers may used in performing the presentinvention. Metallocenes can be characterized generally as coordinationcompounds incorporating one or more cyclopentadienyl (Cp) groups (whichmay be substituted or unsubstituted and may be the same or different)coordinated with a transition metal through Π bonding. The Cp groups mayalso include substitution by linear, branched or cyclic hydrocarbylradicals and desirably cyclic hydrocarbyl radicals so as to form othercontiguous ring structures, including, for example indenyl, azulenyl andfluorenyl groups. These additional ring structures may also besubstituted or unsubstituted by hydrocarbyl radicals and desirablyC1-C20 hydrocarbyl radicals. Metallocene compounds may be combined withan activator and/or cocatalyst (as described in greater detail below) orthe reaction product of an activator and/or cocatalyst, such as forexample methylaluminoxane (MAO) and optionally an alkylation/scavengingagent such as trialkylaluminum compound (TEAl or optionally TiBAl).Various types of metallocenes are known in the art that may besupported. Typical support may be any support such as talc, an inorganicoxide, clay, and clay minerals, ion-exchanged layered compounds,diatomaceous earth, silicates, zeolites or a resinous support materialsuch as a polyolefin. Specific inorganic oxides include silica andalumina, used alone or in combination with other inorganic oxides suchas magnesia, titania, zirconia and the like. Non-metallocene transitionmetal compounds, such as titanium tetrachloride, are also incorporatedinto the supported catalyst component. The inorganic oxides used assupport are characterized as having an average particle size rangingfrom 30-600 microns, desirably from 30-100 microns, a surface area of50-1,000 square meters per gram, desirably from 100-400 square metersper gram, a pore volume of 0.5-3.5 cc/g, desirably from about 0.5-2cc/g.

[0023] As used herein unless otherwise indicated, “metallocene” includesa single metallocene composition or two or more metallocenecompositions. Metallocenes are typically bulky ligand transition metalcompounds generally represented by the formula:

[L]_(m)M[A]_(n)

[0024] where L is a bulky ligand, A is a leaving group, M is atransition metal and m and n are such that the total ligand valencycorresponds to the transition metal valency.

[0025] The ligands L and A may be bridged to each other, and if twoligands L and/or A are present, they may be bridged. The metallocenecompounds may be a full-sandwich compounds having two or more ligands Lwhich may be cyclopentadienyl ligands or cyclopentadiene derived ligandsor half-sandwich compounds having one ligand L, which is acyclopentadienyl ligand or cyclopentadienyl derived ligand. Thetransition metal atom may be a Group 4, 5, or 6 transition metal and/ora metal from the lanthanide and actinide series. Zirconium, titanium,and hafnium are desirable. Other ligands may be bonded to the transitionmetal, such as a leaving group, such as but not limited to hydrocarbyl,hydrogen or any other univalent anionic ligand.

[0026] Preferred metallocenes are unbridged metallocenes that may bedescribed by the general formula:

Cp₂M′Q_(n)

[0027] where Cp is a cyclopentadienyl group, each being the same ordifferent and which may be substituted or unsubstituted; M′ is atransition metal; and Q is a halogen, an alkyl, or hydrocarbyl havingfrom 1-20 carbon atoms; and n+2 equals the valence of the transitionmetal. Preferably M′ is a group IVB metal, more preferably zirconium ortitanium, and most preferably titanium, which has a valence of 4.Preferably, Q is a halogen or alkyl, more preferably chlorine or methyl,and most preferably chlorine. A preferred unbridged metallocene is bis(cyclopentadienyl) titanium dichloride, also referred to as titanocenedichloride.

[0028] A bridged metallocene, for example, may be described by thegeneral formula:

RCpCp′MeQn.

[0029] where Me is a transition metal element; Cp and Cp′ each is acyclopentadienyl group, each being the same or different and which canbe either substituted or unsubstituted; Q is an alkyl or otherhydrocarbyl or a halogen group; n is a number and may be within therange of 1-3; and R is a structural bridge extending between thecyclopentadienyl rings. Metallocene catalysts and metallocene catalystssystems that produce isotactic polyolefins are disclosed in U.S. Pat.Nos. 4,794,096 and 4,975,403 which are incorporated by reference herein.These patents disclose chiral, stereorigid metallocene catalysts thatpolymerize olefins to form isotactic polymers and are especially usefulin the polymerization of highly isotactic polypropylene.

[0030] Suitable metallocene catalysts are disclosed in, for example,U.S. Pat. Nos. 4,530,914; 4,542,199; 4,769,910; 4,808,561; 4,871,705;4,933,403; 4,937,299; 5,017,714; 5,026,798; 5,057,475; 5,120,867;5,132,381; 5,155,180; 5,198,401; 5,278,119; 5,304,614; 5,324,800;5,350,723; 5,391,790; 5,436,305; 5,510,502; 5,145,819; 5,243,001;5,239,022; 5,329,033; 5,296,434; 5,276,208; 5,672,668; 5,304,614,5,374,752; 5,510,502; 4,931,417; 5,532,396; 5,543,373; 6,100,214;6,228,795; 6,124,230; 6,114,479; 6,117,955; 6,087,291; 6,140,432;6,245,706; 6,194,341; and EP 549 900; 576 970; and 611 773; and WO97/32906; 98/014585; 98/22486; and 00/12565, each of which isincorporated by reference herein in its entirety.

[0031] Metallocenes may be used in combination with some form ofactivator in order to create an active catalyst system. The term“activator” is defined herein to be any compound or component, orcombination of compounds or components, capable of enhancing the abilityof one or more metallocenes to polymerize olefins to polyolefins.Alklyalumoxanes such as methylalumoxane (MAO) are commonly used asmetallocene activators. Generally alkylalumoxanes contain about 5 to 40of the repeating units and may be described by the general formulas:

[0032] where R is a C1-C8 alkyl including mixed alkyls. Particularlydesirable are the compounds in which R is methyl. Alumoxane solutions,particularly methylalumoxane solutions, may be obtained from commercialvendors as solutions having various concentrations. There are a varietyof methods for preparing alumoxane, non-limiting examples of which aredescribed in U.S. Pat. Nos. 4,665,208, 4,952,540, 5,091,352, 5,206,199,5,204,419, 4,874,734, 4,924,018, 4,908,463, 4,968,827, 5,308,815,5,329,032, 5,248,801, 5,235,081, 5,103,031 and EP-A-0 561 476, EP 0 279586, EP-A-0 594 218 and WO 94/10180, each incorporated herein byreference.

[0033] Ionizing activators may also be used to activate metallocenes.These activators are neutral or ionic, or are compounds such astri(n-butyl)ammonium tetrakis(pentaflurophenyl)borate, which ionize theneutral metallocene compound. Such ionizing compounds may contain anactive proton, or some other cation associated with, but not coordinatedor only loosely coordinated to, the remaining ion of the ionizingcompound. Combinations of activators may also be used, for example,alumoxane and ionizing activators in combinations. Suitable ionicactivators are disclosed in, for example, WO 94/07928, EP-A-0 277 003,EP-A-0 277 004, U.S. Pat. No. 5,198,401, WO-A-92/00333, EP-A-0 426 637,EP-A-0 573 403, EP-A-0 520 732, EP-A-0 495 375, EP-A-O 500 944, EP-Al-0570, U.S. Pat. No. 5,643,847, U.S. patent application Ser. No. 09184358,filed Nov. 2, 1998 and U.S. patent application Ser. No. 09184389, filedNov. 2, 1998, all fully incorporated herein by reference.

EXAMPLES

[0034] The invention having been generally described, the followingexamples are given as particular embodiments of the invention and todemonstrate the practice and advantages thereof. It is understood thatthe examples are given by way of illustration and are not intended tolimit the specification or the claims to follow in any manner. Thefollowing is a summary of the polymerization conditions used in theexamples: wt. of ZN catalyst: 10 mg wt. of metallocene: 0, 3, 5, or 10mg amount of TEA1 (co-catalyst): 0.25 mmole/l Hydrogen: 2, 4, 6 or 8SLPM Ethylene: 4 or 8 SLPM Temp.: 80° C. Time: 120 minutes Pressure 125psi

[0035] The Autoclave Engineer reactor used for these polymerizations ofethylene has a four liter capacity and is fitted with four mixingbaffles with two opposed pitch mixing propellers. Ethylene and hydrogenare introduced to the reactor vessel via Teledyne-Hastings Raydist massflow controllers while a downloaded backpressure regulator kept theinternal reaction pressure constant. The reaction temperature ismaintained by steam and cold water in the reactor jacket using a KammerValve linked to a Barber-Coleman controller.

Example 1

[0036] The reactor was filled with 2 liters of hexane and thetemperature was increased to 80° C. with agitation. 10 mg of a typicalZeigler-Natta catalyst was suspended in mineral oil and precontactedwith 2 ml TEA1 solution (0.25 mmole/1 in hexane) in a dry box. Hydrogenand ethylene were admitted to the reactor at the flow rates of 2standard liters per minute (SLPM) and 8 SLPM, respectively, and thereactor backpressure was maintained at 125 psi. The ZNcatalyst-cocatalyst mixture was charged into the reactor. The reactortemperature and pressure were maintained for one hour. After one hour ofpolymerization, 3 mg of a metallocene catalyst, specifically titanocenedichloride, were charged into the reactor and polymerization wascontinued for another hour under the same reactor temperature andpressure. After transferring the polymer fluff slurry from the reactorto a flask, the solvent was removed and the polymer was collected as dryfluff.

Example 2

[0037] The procedure of Example 1 was followed except that 5 mg oftitanocene dichloride were charged into the reactor.

Example 3

[0038] The procedure of Example 1 was followed except that 10 mg oftitanocene dichloride were charged into the reactor.

Comparative Example 1

[0039] The procedure of Example 1 was followed except no titanocenedichloride was charged into the reactor.

Example 4

[0040] The procedure of Example 1 was followed except that the hydrogenand ethylene flow rates were at 4 SLPM and 8 SLPM, respectively.

Example 5

[0041] The procedure of Example 4 was followed except that 10 mg oftitanocene dichloride were charged into the reactor.

Comparative Example 2

[0042] The procedure of Example 4 was followed except no titanocenedichloride was charged into the reactor.

Example 6

[0043] The procedure of Example 1 was followed except that the hydrogenand ethylene flow rates were at 6 SLPM and 8 SLPM, respectively.

Example 7

[0044] The procedure of Example 6 was followed except that 10 mg oftitanocene dichloride were charged into the reactor.

Comparative Example 3

[0045] The procedure of Example 6 was followed except no titanocenedichloride was charged into the reactor.

Example 8

[0046] The procedure of Example 1 was followed except that the hydrogenand ethylene flow rates were at 16 SLPM and 8 SLPM, respectively, and 5mg of titanocene dichloride were charged into the reactor.

Example 9

[0047] The procedure of Example 8 was followed except that 10 mg oftitanocene dichloride were charged into the reactor.

Comparative Example 4

[0048] The procedure of Example 8 was followed except no titanocenedichloride was charged into the reactor.

[0049] Table I shows the effect of the addition of metallocene compoundon a conventional supported Ziegler-Natta catalyst with TEA1 only as aco-catalyst., wherein M:Z-N is the weight ratio of metallocene catalystto Zeigler-Natta catalyst and yield is polymer yield in grams. The datademonstrates that such use of a metallocene catalyst with aZeigler-Natta catalyst yields bimodal polyethylene with broadermolecular weight distribution, decreased melt index, higher molecularweight, and enhanced melt strength. FIGS. 1-7 are GPC curves for thepolymer produced in Examples 1-7, respectively. FIG. 8 is a GPC curvefor the polymer produced in Comparative Example 2.

[0050] While the preferred embodiments and examples of the inventionhave been shown and described, modifications thereof can be made by oneskilled in the art without departing from the spirit and teachings ofthe invention. Reactor design criteria, pendant polymer processingequipment, and the like for any given implementation of the inventionwill be readily ascertainable to one of skill in the art based upon thedisclosure herein. The embodiments and examples described herein areprovided for illustration and are not intended to be limiting. Manyvariations and modifications of the invention disclosed herein arepossible and are within the scope of the invention. Accordingly, thescope of protection is not limited by the description set out above, butis only limited by the claims which follow, that scope including allequivalents of the subject matter of the claims. TABLE 1 EXAMPLE M:Z-N(w/w) H2/C2 Yield M15 HLM1 HLM1/M15 Mn Mw Mz Mw/Mn Hz/Mw 1 0.3 0.25 2240.427 5.38 12.60 30750 215674 1128283 7.0 5.2 2 0.5 0.25 321 0.673 8.3612.42 28944 216840 1270149 7.5 5.9 3 1 0.25 335 0.111 2.36 21.26 30480362817 2626799 11.9 7.2 Comparative 1 0 0.25 273 1.58 19.21 12.16 25551168429 925964 6.6 5.5 4 0.3 0.5 209 2.33 27.79 11.93 21894 151633 8785126.9 5.8 5 1 0.5 149 0.637 8.55 13.42 21959 200202 1463609 9.1 7.3Comparative 2 0 0.5 186 5.62 49.82 8.86 17653 124943 796964 7.1 6.4 60.3 0.75 111 5.81 88.94 15.31 17704 121467 770417 6.9 6.3 7 1 0.75 1484.56 50.52 11.08 18329 130716 824378 7.1 6.3 Comparative 3 0 0.75 1159.66 108.2 11.20 15832 100401 608116 6.3 6.1 8 0.5 2.0 47 115 — — 922661656 616100 6.7 10.0 9 1 2.0 53 120.9 — — 9015 59731 574208 6.6 9.6Comparative 4 0 2.0 49 65.95 — — 11394 61840 374153 5.4 6.1

What is claimed is:
 1. A polyethylene polymerization process,comprising: (a) polymerizing polyethylene in the presence aZeigler-Natta catalyst for a period of time; and (b) adding ametallocene catalyst and continuing the polymerization for an additionalperiod of time.
 2. The process of claim 1 wherein the polymerizationprocess produces polyethylene having a bimodal molecular weightdistribution.
 3. The process of claim 2 wherein steps (a) and (b) areperformed in a single reactor vessel.
 4. The process of claim 2 whereinstep (a) is performed in a first reactor vessel and step (b) isperformed in a second, downstream reactor vessel.
 5. The process ofclaim 4 wherein the reactor vessels are continuously stirred tankreactors.
 6. The process of claim 1 wherein the metallocene catalyst isunbridged.
 7. The process of claim 1 wherein the metallocene catalystcomprises a bulky ligand transition metal compound generally representedby the formula: [L]_(m)M′[A]_(n) where L is a bulky ligand, A is aleaving group, M′ is a transition metal, and m+n equals the valence ofthe transition metal.
 8. The process of claim 7 wherein the bulky ligandtransition metal compound is generally represented by the formula:Cp₂M′Q_(n) where Cp is a cyclopentadienyl group, each being the same ordifferent and which may be substituted or unsubstituted; M′ is atransition metal; and Q is a halogen, an alkyl, or hydrocarboxyl; andn+2 equals the valence of the transition metal.
 9. The process of claim8 wherein M′ is titanium, Q is chlorine, and n is
 2. 10. The process ofclaim 1 wherein the metallocene catalyst comprises bis(cyclopentadienyl)titanium dichloride.
 11. The process of claim 5 wherein the metallocenecatalyst comprises bis(cyclopentadienyl) titanium dichloride.
 12. Theprocess of claim 1 wherein the Zeigler-Natta catalyst comprises atransition metal compound generally represented by the formula: MR⁺ _(x)where M is a transition metal, R⁺ is a halogen or a hydrocarboxyl, and xis the valence of the transition metal.
 13. The process of claim 12wherein M is a group IVB metal and R⁺ is chlorine, bromine, or alkoxy.14. The process of claim 13 wherein M is titanium and R⁺ is chlorine orethoxy.
 15. The process of claim 1 wherein the transition metal compoundis TiCl₄, TiBr₄, Ti(OC₂H₅)₃Cl, Ti(OC₃H₇)₂Cl₂, Ti(OC₆H₁₃)₂Cl₂,Ti(OC₂H₅)₂Br₂, or Ti(OC₁₂H₂₅)Cl₃.
 16. The process of claim 11 whereinthe Zeigler-Natta catalyst comprises TiCl₄.
 17. A polymer produced bythe polymerization process of claim
 16. 18. An article of manufacturecomprising the polymer of claim
 17. 19. A bimodal polyethylenepolymerization process comprising using a metallocene catalyst to adjustthe hydrogen response of a Ziegler-Natta catalyst.
 20. The process ofclaim 19 wherein the polymerization is carried out in a single reactor.21. The process of claim 19 wherein the polymerization is carried out intwo or more reactors in series.
 22. The process of claim 21 wherein theZeigler-Natta catalyst is added to a first reactor and the metallocenecatalyst is added to a downstream reactor.
 23. The process of claim 19wherein the Zeigler-Natta catalyst and metallocene catalyst are added toa first reactor.
 24. The process of claim 23 wherein the Zeigler-Nattacatalyst and metallocene catalyst are premixed prior to addition to thefirst reactor.